Motor drives and other forms power conversion systems convert electrical power from one form to another and may be employed in a variety of applications such as powering an electric motor using power converted from a single or multiphase AC input source, converting AC power from a wind driven generator to supply power to a grid, etc. Power converters may include multiple stages for different types of conversion applications, such as AC/DC/AC drives for electric motors having a pulse width modulated (PWM) active current source rectifier in which AC input power is selectively switched to create a DC output bus voltage from which a load is driven by a PWM controlled inverter stage. This type of converter is particularly useful in driving electric motors in industrial applications requiring variable speed control with varying motor load situations.
In the controlled switching of the input and output power in one or more converter stages, high voltages and currents are experienced by the power switches, wherein semiconductor-based switches such as silicon controlled rectifiers (SCRs), gate turn-off thyristors (GTOs), gate commutated thyristors (GCTs such as integrated gate commutated thyristors (IGCTs) or symmetrical gate commutated thyristors (SGCTs)), etc. are typically used to switch the power at relatively high frequencies. The control terminals or gates of these switching devices require gate driver circuits to generate the switch signals for actuating the power switch for controlled operation in power conversion applications, wherein GCTs typically have an external gate driver and IGCTs have a driver circuit integral to the GCT power switch. In medium or high voltage power converters, the gate driver circuit needs to be operated with power that is isolated from the system ground since the input and output voltages may be several thousand volts or more. Conventional power converters often employ separate isolated DC voltage sources for powering the gate driver circuits. However, these driver power sources and voltage transformers occupy valuable space and add significant cost in power conversion systems, particularly for medium or high voltage systems operated with GCT or IGCT type switches. Thus, there remains a need for improved power conversion systems and gate driver power sources by which isolated gate driver power can be provided in a cost efficient manner without adding large external power sources.